Video signals can be classified as either progressive or interlaced. In a progressive video signal, video frames are displayed sequentially, with horizontal lines of pixels that make up the frame being displayed line by line from top to bottom. In an interlaced signal, each frame is split into two fields which are displayed in turn. One field contains the top line of the frame and every second subsequent line, whilst the other field contains the second line from the top and every second subsequent line. When an interlaced video signal is displayed, the fields are displayed sequentially, with the second field of each frame being displayed very shortly after the first field.
Interlaced video relies on the fact that pixels of a display apparatus continue to emit light for a short period (known as the decay period) after their stimulus has been removed to create the illusion that one complete frame is being displayed at a time, as the first field displayed remains visible for a period whilst the second field is displayed.
Interlaced signals can be displayed without difficulty on a cathode ray tube (CRT) based display apparatus such as a CRT television set. However, more modern technologies such as liquid crystal display (LCD) and plasma screens cannot directly display interlaced video signals, and thus a de-interlacing process, in which the interlaced video signal is converted to a progressive video signal, is required to display interlaced video signals on such apparatus.
The de-interlacing process used depends upon the original format of the interlaced video signal. For example, the interlaced video signal may have been recorded in interlaced format originally (true interlaced), or may have been recorded in a progressive format and subsequently converted into interlaced format for broadcast. Different de-interlacing modes are required to de-interlace these different types of interlaced video signals.
The situation is further complicated by the fact that different interlacing techniques (known as cadences) may be used to convert a progressive video signal to interlaced format. In one example, which is illustrated in FIG. 1, a progressive frame 10 is converted into first and second interlaced fields 12 and 14, with the first field 12 including the top line and every subsequent second line of the progressive frame 10, whilst the second field 14 includes the second line from the top of the progressive frame 10 and every subsequent second line. However, different cadences are employed for different purposes. For example, films are typically recorded at 24 frames per second in progressive format, whereas the National Television System Committee (NTSC) television standard requires a display rate of 60 fields per second. Thus, to convert a film recorded at 24 frames per second in progressive format to interlaced NTSC format a 3:2 cadence is used to spread the 24 frames in each second of the original progressive signal over 60 fields. This is illustrated in FIG. 2, which shows a two-frame progressive video sequence 20 being interlaced using a 3:2 cadence. A first frame 22 of the sequence is converted into three interlaced fields 24, 26 and 28, one of which (the third field 28 in this example) is a duplicate of another of the fields (the first field 24 in this example), and the second frame 30 is converted into two different interlaced fields 32 and 34. Alternate frames of the progressive video sequence 20 are converted into three or two interlaced fields, such that for every 24 frames of the original progressive video signal 60 interlaced fields are produced.
Alternative cadences are also prevalent, such as 2:2:2:2:2:2:2:2:2:2:2:3 or 2:2, which are used for converting 24 frames per second progressive films to 50 fields per second interlaced Phase Alternate Line (PAL) standard video.
A problem arises in de-interlacing an interlaced video signal when thu interlaced video signal contains different formats of interlaced video. For example, a television broadcast may include program segments which were originally recorded in interlaced format and advertisement segments which were originally recorded in progressive format and converted to interlaced format. Additionally, different cadences may have been used for different interlaced segments of the signal. De-interlacing modes designed for true interlaced content (i.e., video signals which were recorded in interlaced format, rather than being converted from a progressive format) do not perform well on converted progressive content. Thus, different de-interlacing modes are required to achieve satisfactory de-interlacing of each different type of interlaced video signal, and it is important that any decoding or display apparatus is able to switch between these different modes at run-time.
Techniques exist to allow run-time switching between de-interlacing modes, typically involving calculating a metric indicative of the difference between adjacent fields of a video sequence, which metric is then compared to a threshold to determine the type of interlaced sequence that is present. A suitable de-interlacing mode is then selected, and the decoding or display apparatus “locks in” to that mode to avoid switching between de-interlacing modes when a static sequence of images is detected. However, in some cases the selected de-interlacing mode may not be appropriate for the whole of a video sequence, which can lead to poor performance and the appearance of artifacts in the sequence when it is displayed.